Abstract: In order to develop renewable energy,the offshore wind energy technology has been a main research field in Europe.A foundation with sufficient stiffness to resist long-term wind and wave load to insure the serviceability of wind energy converter is needed.The monopile foundation,the steel pipe pile with a large diameter,is one of well adopted foundations in the existing and planned offshore wind parks.The usually used design method,p-y curves method,is unsuitable for estimating the deformation response of the large-diameter monopile under long-term cyclic lateral load.This paper aims to present a calculation method for estimating the lateral deformation response of a single pile in sandy soil under cyclic lateral load based on the cyclic triaxial test results.The degradation of secant soil stiffness due to the accumulation of plastic strain under cyclic loading is formulated.Through this degradation stiffness model,the accumulated deformation of a single pile can be quantified using a finite element model with three calculation steps.The proposed method is applicable for uniform or layered subsoil in a preliminary design.